Resistance to anticoagulants in rats (<em>Rattus norvegicus</em>) in sewers in an urban area

Control of rats in sewers is, though of varying intensity, common practice in a majority of Danish municipalities and bromadiolone is the most preferred active ingredient. The results of sewer rat control is very difficult to register and very little is known about resistance among sewer rats. Therefore a project for studying resistance levels in sewer rats was undertaken in an urban area of a municipality in southern Zealand from where surface rats had been tested during the foregoing years. Rats resistant to difenacoum represented the highest level of resistance in the municipality. Rats were primarily tested for possible resistance to coumatetralyl, bromadiolone and difenacoum by Blood Clotting Response tests. Feeding test was used in tests for resistance to difethialone. A total of 24 rats trapped in sewers at 15 locations were tested. Resistance to bromadiolone was found among rats from all sewer rat locations and resistance to difenacoum was found in rats from 11. No rats were found resistant to difethialone. The findings of rats resistant to difenacoum in a majority of the sewer locations clearly indicate that use of bromadiolone is insufficient for an effective control. Rodenticides belonging to the most potent group of anticoagulants are to be recommended for control of sewer rats in the investigated area. As in this case study resistance to difenacoum has been found in surface living rats in many other municipalities. The normal and widespread use of bromadiolone for control of sewer rats has therefore to be revised.